Plastics such as polyesters are typically produced from petrochemical sources by well-known synthetic means. These petrochemical-based polymers take centuries to degrade after disposal. Concern over plastic waste accumulation in landfills has resulted in the recent movement toward using biodegradable polymers instead.
Synthetic biodegradable polymers have not enjoyed great success in the marketplace due to their high production cost. However, advances in biotechnology have led to less expensive means of production. Specifically, biodegradable aliphatic copolyesters are now produced by large-scale bacterial fermentation. Collectively termed polyhydroxyalkanoates or PHAs, these polymers may be synthesized in the bodies of natural or recombinant bacteria fed with glucose in a fermentation plant. Like their petrochemical precursors, the structural, and in turn mechanical, properties of PHAs may be customized to fit the specifications of the desired end product. However, unlike their petrochemical precursors, PHAs degrade both aerobically and anaerobically.
PHAs are enormously versatile, and as many as 100 different PHA structures have been identified. PHA structures may vary in two ways. First, PHAs may vary according to the structure of the R-pendant groups, which form the side chain of hydroxyalkanoic acid not contributing to the PHA carbon backbone. Second, PHAs may vary according to the number and types of units from which they are derived. For example, PHAs may be homopolymers, copolymers, and terpolymers. These variations in PHA structure are responsible for the variations in their physical characteristics. In turn, the variations in physical characteristics are what make generic application of the known PHA extraction processes inefficient.
Solvent extraction has been described as one method of extracting PHAs from bacteria and plants. However, solvent extraction of structurally flexible PHA polymers is challenging, because these PHAs crystallize slowly and tend to stick together to form a gel as the extraction solvent is removed. Gel formation is undesirable, because it requires the additional, costly steps of driving the solvent out of the gel, and flaking the remaining structurally flexible PHAs, to obtain a neat polymer. Several two-solvent extraction processes purport to circumvent the gel formation problem. However, two-solvent extractions may be more costly than single-solvent ones, because they generally require twice as much solvent and may preclude recycling of solvents without additional solvent processing, such as separating the two solvents.
For the foregoing reasons, there is a need for a simple and more economical process for extracting structurally flexible PHA polymers from bacteria and plants. Such a process would preferably involve a single recyclable main solvent that is preferably environmentally friendly.